In order to protect semiconductor elements mounted on a lead-frame, substrate, package or the like, and their peripheral electrode wirings and the like from an external environment, resin encapsulation that a resin is poured around them to encapsulate them is conducted. The resin encapsulation is conducted for the purpose of protecting semiconductor elements such as IC and LSI, and wirings such as bonding wires, inner leads from an external environment and improving handling property upon mounting.
As a resin for encapsulation, is mainly used an epoxy type encapsulant. In the epoxy type encapsulant, is generally an epoxy compound (epoxy resin) having at least 2 epoxy groups in its molecule. When a curing agent is added to this compound, and the resultant mixture is heated, a cured product excellent in chemical resistance and mechanical strength is obtained. As the epoxy type encapsulant, is used an epoxy type resin composition, in which an epoxy resin is used as a principal agent, and secondary agents selected from a curing agent, a curing accelerator, a plasticizer, a filler, a coupling agent, a flame retardant auxiliary, a colorant, a parting agent, an ion scavenger and the like are suitably added thereto.
Methods for the resin encapsulation include a transfer molding method, a dipping method, a potting method, a powder fluidization dipping method, etc. Among these, most of packages are resin-encapsulated by the transfer molding method because it is suitable for mass production. In order to conduct resin encapsulation with the epoxy type encapsulant by the transfer molding method, a process comprising forming the epoxy type encapsulant into tablets (B-stage solid bodies), inserting the epoxy tablets into a mold heated to a high temperature and pressurizing the mold to encapsulate a semiconductor chip or module installed in the mold in advance is generally adopted. The epoxy resin material forms a resin-encapsulated part cured by heating. A process for conducting resin encapsulation by injection molding using the epoxy type encapsulant has been recently proposed.
A resin material for encapsulation is required to have excellent electrical insulating property, mechanical properties, heat resistance, chemical resistance and dimensional stability and also to have properties such as moisture resistance, stress-relieving ability, circuit-shielding ability, light-screening ability and heat-radiating ability. For example, a semiconductor element such as IC is a minute circuit in itself, so that resin encapsulation must be conducted in a state that insulating property between terminals has been retained. Therefore, the resin material for encapsulation requires having electrical insulating property. Since the circuit of the semiconductor element is liable to be broken by water and ionic impurities, the resin material for encapsulation is also required to be low in coefficient of moisture absorption (coefficient of water absorption).
In order to obtain a resin material for encapsulation satisfying such various required properties, there have heretofore been proposed, for example, methods of selecting the kinds of a resin and a filler or raising the content of the filler. More specifically, there have been proposed an epoxy resin composition, in which spherical silica having a specific particle size distribution is incorporated for the purpose of making the mold shrinkage factor of a cured product low to permit precise injection molding (Japanese Patent Application Laid-Open No. 11-323097; U.S. Pat. No. 5,064,881), a liquid semiconductor encapsulant with amorphous silica powder having excellent dispersibility filled into a liquid epoxy resin (Japanese Patent Application Laid-Open No. 2002-212398), an epoxy resin composition for encapsulation of semiconductors, in which partially spherical silica having a vitrification rate of 10 to 95% by weight is incorporated into an epoxy resin for the purpose of making a coefficient of linear expansion high to obtain a cured product low in water absorption property and rate of crack initiation (Japanese Patent Application Laid-Open No. 2000-063636), a composition with 40 to 85% by mass of spherical silica having an average particle diameter of 2 to 10 μm and an extremely small specific surface area contained in a liquid cyclic skeletal-containing epoxy resin for the purpose of obtaining a cured product excellent in resistance to thermal adhesion, moisture resistance and flexural modulus (Japanese Patent Application Laid-Open No. 2001-226562), a resin composition for encapsulation, in which fused silica and crystalline silica are incorporated into a dicyclopentadiene type epoxy resin in combination for the purpose of retaining or improving crack resistance upon solder reflowing even when an after cure step is omitted (Japanese Patent Application Laid-Open No. 2002-249546), and the like.
However, the conventional resin materials for encapsulation have involved a problem that they cannot sufficiently cope with ESD troubles such as an electrostatic discharge phenomenon by static electricity and electrostatic destroy caused by this electrostatic discharge phenomenon. With the advancement of high-density and fine-pitch modules in electronic devices such as semiconductor elements, such an electronic device tends to be charged by the influence of frictional electrification at a resin-encapsulated part when the resin-encapsulated part has a surface resistivity exceeding 1013Ω/□. The electronic device charged and stored with static electricity is damaged by discharge of the static electricity and electrostatically adsorbs suspended dust in the air. On the other hand, an electronic device, which has a resin-encapsulated part having a surface resistivity lower than 105Ω/□ may suffer from trouble in some cases by a strong current or high voltage generated upon discharge of static electricity because a charge transfer speed in the resin-encapsulated part is too fast. In addition, when the surface resistivity of the resin-encapsulated part is too low, it is impossible to retain electrical insulating property.
In the electronic device resin-encapsulated with the resin material for encapsulation, the problem of the ESD troubles by the resin-encapsulated part has heretofore not been sufficiently recognized by those skilled in the art. There has been no proposal for solving the problem of the ESD troubles in the resin material for encapsulation. With the advancement of high-density and fine-pitch modules in electronic devices, the resin materials used in the technical field of these devices also have an important object of enabling them to cope with the ESD troubles from the viewpoints of sufficiently protecting the electronic devices from the electrostatic trouble and repelling dust to retain high cleanness.
The methods of selecting the kinds of an epoxy resin and a filler and the particle diameter and particle size distribution of the filler like the conventional epoxy type encapsulants cannot cope with the ESD troubles under the circumstances. In order to cope with the ESD troubles, it is necessary to control the surface resistivity of the resin-encapsulated part within a range of 105 to 1013Ω/□ that is a semiconductive region. However, it has been extremely difficult to develop a resin material for encapsulation, by which the surface resistivity of a resin-encapsulated part can be strictly and stably controlled within the range of 105 to 1013Ω/□ that is a semiconductive region while retaining properties such as electrical insulating property, mechanical properties, heat resistance, chemical resistance, dimensional stability and moisture resistance that are required of the resin-encapsulated part.
As a method for lowering the surface resistivity of the resin-encapsulated part, is considered a method of incorporating an antistatic agent or conductive filler into a resin material for encapsulation. In the method of incorporating the antistatic agent into the resin material for encapsulation, however, the antistatic agent present on the surface of the resin-encapsulated part is easily removed by washing or friction to lose the antistatic effect. When the amount of the antistatic agent incorporated is increased to make the antistatic agent easy to bleed from the interior of the resin-encapsulated part to the surface thereof, the antistatic effect can be somewhat sustained, but dust adheres to the surface of the resin-encapsulated part due to the antistatic agent bled, and the electronic device and an environment are contaminated by dissolving-out or volatilization of the antistatic agent. When the antistatic agent is incorporated in a great amount, the heat resistance of the resin-encapsulated part is lowered.
On the other hand, according to the method of incorporating the conductive filler having a volume resistivity lower than 102 Ω·cm, such as conductive carbon black or carbon fiber, into the resin material for encapsulation, the surface resistivity of the resin-encapsulated part is greatly changed even by a slight change in the proportion of the conductive filler incorporated and a slight change in molding conditions, since a difference in electric resistivity between the resin component and the conductive filler is great. Therefore, the method of simply incorporating the conductive filler is extremely difficult to strictly and stably control the surface resistivity of the resin-encapsulated part to a desired value within the range of 105 to 1013Ω/□. In addition, according to the method of incorporating the conductive filler, the surface resistivity of the resin-encapsulated part is liable to great variation in different localities. The resin-encapsulated part showing great variation in surface resistivity has a possibility that electrical insulating property between terminals of a semiconductor element may be impaired, since places too high in surface resistivity and places too low in surface resistivity are present in a state mixed in the resin-encapsulated part. In addition, such a resin-encapsulated part cannot sufficiently protect the electronic device from the ESD troubles.